The present invention relates to a method of inhibiting rust from arising on a metallic substrate in case of coating the metallic substrate with a thermosetting fluorine-containing resin powder coating composition.
It is known that a coated article being excellent in weather resistance can be obtained by applying a thermosetting fluorine-containing resin powder coating composition on a metallic substrate (JP-B-6-104792, JP-A-6-345822, JP-A-6-184243, etc.).
Also it is described in JP-A-6-184243 that a fluorine-containing resin copolymer containing no chlorotrifluoroethylene is excellent particularly in yellowing resistance of a coating film at a weather resistance test.
As mentioned above, a priority has been given to characteristics of a coating film, and with respect to a relation between a substrate and a coating film, only adhesion between them has been taken up as a problem to be solved.
However though a substrate coated with a thermosetting fluorine-containing resin powder coating composition exhibits excellent gloss retention and a minimum change of color difference because of excellent weather resistance of the fluorine-containing resin, the composition has a problem with rust on a contact surface of a metallic substrate in outdoor use. Particularly when the fluorine-containing powder coating composition is directly coated on a metallic substrate to form a coating film without using a primer, rusting arises remarkably.
This problem with rusting of a metallic substrate coated with a thermosetting fluorine-containing resin powder coating composition has not yet been solved.
The present invention relates to the method of inhibiting rust of a metallic substrate by coating the metallic substrate with a thermosetting fluorine-containing resin powder coating composition which does not contain essentially chlorine atom.
The thermosetting fluorine-containing resin powder coating composition to be used in the method of the present invention basically comprises a thermosetting fluorine-containing resin and a curing agent, the both of which contain no chlorine essentially.
Examples of the thermosetting fluorine-containing resin containing no chlorine are, for instance, copolymers which comprise a fluoroolefin unit containing no chlorine atom as an essential component and have a crosslinkable reactive group.
Examples of the fluoroolefin containing no chlorine atom are, for instance, tetrafluoroethylene, monofluoroethylene, trifluoroethylene, vinylidene fluoride, hexafluoropropylene, pentafluoropropylene, and the like. The fluoroolefin can be optionally selected depending on characteristics required on a coating film and a combination with other copolymer component and curing agent. Those fluoroolefins can be used in combination of two or more thereof. Particularly preferable fluoroolefins are perfluoroolefins such as tetrafluoroethylene and hexafluoropropylene from the viewpoint of copolymerizability with vinyl ether and/or vinyl ester monomers.
Examples of the crosslinkable reactive group of the thermosetting fluorine-containing resin to be used in the present invention are hydroxyl group, carboxyl group, amido group, amino group, mercapto group, glycidyl group, isocyanate group, and the like. Examples of a method for introducing such a crosslinkable reactive group into the copolymer are a method of copolymerizing monomer having a crosslinkable reactive group, a method of decomposing a part of the copolymer, a method of reacting a reactive group of the copolymer with a compound giving a crosslinkable reactive group thereto, and the like.
Examples of the comonomer which can introduce a suitable crosslinkable reactive group by copolymerization are, for instance, monomers having hydroxyl group or a group which can be converted to hydroxyl group and having a double bond copolymerizable with the fluoroolefin. Examples thereof are one or a mixture of two or more of hydroxyalkyl vinyl ethers such as hydroxyethyl vinyl ether, hydroxypropyl vinyl ether, hydroxybutyl vinyl ether, hydroxyisobutyl vinyl ether and hydroxycyclohexyl vinyl ether; esters of vinyl alcohol and hydroxyalkylcarboxylic acid such as vinyl hydroxyacetate, vinyl hydroxypropionate, vinyl hydroxybutyrate, vinyl hydroxyvalerate, vinyl hydroxyisobutyrate and vinyl hydroxycyclohexanecarboxylate; hydroxyalkyl allyl ethers such as hydroxyethyl allyl ether, hydroxypropyl allyl ether, hydroxybutyl allyl ether, hydroxyisobutyl allyl ether and hydroxycyclohexyl allyl ether; hydroxyalkyl allyl esters such as hydroxyethyl allyl ester, hydroxypropyl allyl ester, hydroxybutyl allyl ester, hydroxyisobutyl allyl ester and hydroxycyclohexyl allyl ester; hydroxyalkyl esters of acrylic acid or methacrylic acid such as 2-hydroxyethyl acrylate, hydroxypropyl acrylate, 2-hydroxyethyl methacrylate and hydroxypropyl methacrylate; partly fluorine-substituted compounds thereof; and the like. One or two or more thereof may be selected and used as a comonomer for introducing hydroxyl group.
Examples of the comonomer having carboxyl group are, for instance, xcex1,xcex2-unsaturated carboxylic acids such as acrylic acid, methacrylic acid, itaconic acid, succinic acid, succinic anhydride, fumaric acid, fumaric anhydride, crotonic acid, maleic acid and maleic anhydride and in addition, fluorine-substituted monomer such as perfluorobutenoic acid, and the like. Also examples of the comonomer having glycidyl group are, for instance, glycidyl (meth)acrylate, glycidyl vinyl ether, glycidyl allyl ether, and the like. Examples of the comonomer having amino group are, for instance, amino alkyl vinyl ether, amino alkyl allyl ether, and the like. Examples of the comonomer having amido group are, for instance, (meth)acrylamide, methylolacrylamide, and the like. Examples of the comonomer having nitrile group are, for instance, (meth)acryronitrile, and the like. Examples of the comonomer having isocyanate group are, for instance, vinyl isocyanate, isocyanate ethyl acrylate, and the like. It is particularly preferable to use a vinyl or allyl ether or ester compound as a comonomer for introducing a crosslinkable reactive group from the viewpoint of excellent copolymerizability with the fluoroolefin.
Example of the method for partly decomposing the copolymer is a method of copolymerizing a monomer having a hydrolyzable ester group after polymerization and then hydrolyzing the copolymer to generate carboxyl group in the copolymer. Also it is possible to form crosslinkage directly by transesterification in a curing reaction without carrying out ester hydrolysis mentioned above.
As the method for reacting the copolymer with a compound giving a crosslinkable reactive group thereto, for example, a method of introducing carboxyl group by reacting a divalent carboxylic acid anhydride such as succinic anhydride with a fluorine-containing copolymer having hydroxyl group, or the like method can be employed suitably.
In addition to the above-mentioned fluoroolefin unit and the unit having a crosslinkable reactive group, a comonomer copolymerizable with those two components can be copolymerized with the fluorine-containing copolymer to be used in the present invention in order to reduce a melting point or a glass transition temperature of the fluorine-containing copolymer for further enhancing workability at coating and imparting physical properties such as proper hardness, flexibility and gloss to the coating film. Example of such an optional comonomer to be used is one which has an unsaturated group being active so as to be copolymerizable with the fluoroolefin and does not lower weather resistance of the coating film remarkably.
Suitable examples of such an optional comonomer are ethylenically unsaturated compounds, for instance, alkyl vinyl ethers such as ethyl vinyl ether, propyl vinyl ether, butyl vinyl ether, isobutyl vinyl ether and cyclohexyl vinyl ether; esters of vinyl alcohol and alkylcarboxylic acid such as vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl valerate, vinyl cyclohexanecarboxylate, vinyl benzoate and vinyl para-t-butylbenzoate; alkyl allyl ethers such as ethyl allyl ether, propyl allyl ether, butyl allyl ether, isobutyl allyl ether and cyclohexyl allyl ether; alkyl allyl esters such as ethyl allyl ester, propyl allyl ester, butyl allyl ester, isobutyl allyl ester and cyclohexyl allyl ester; alkenes such as ethylene, propylene, butylene and isobutylene; acrylic acid and methacrylic acid; esters of acrylic acid or methacrylic acid such as ethyl acrylate, propyl acrylate, butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, isobutyl methacrylate and 2-ethylhexyl methacrylate; partly fluorine-substituted compounds thereof; and the like. Those optional comonomers may be used solely or in combination of two or more thereof.
Among them, vinyl or allyl compounds or alkenes which are excellent in copolymerizability with the fluoroolefin are used preferably as the optional comonomer. In case of using vinyl or allyl alkyl esters or alkyl ethers, a linear, branched or alicyclic alkyl group having about 2 to about 10 carbon atoms can be suitably used as an alkyl group.
From the above-mentioned point of view, examples of the thermosetting fluorine-containing resin are, for instance, tetrafluoroethylene/cyclohexyl vinyl ether/isobutyl vinyl ether/hydroxybutyl vinyl ether copolymer, tetrafluoroethylene/isobutylene/hydroxybutyl vinyl ether copolymer, tetrafluoroethylene/ hexafluoropropylene/ ethylene/ hydroxybutyl vinyl ether copolymer, vinylidene fluoride/tetrafluoroethylene/perfluorobutenoic acid copolymer, and the like.
Thermosetting fluorine-containing resins which can be used suitably in the present invention are copolymers comprising a perfluoroolefin such as tetrafluoroethylene or hexafluoropropylene and a vinyl ether and/or vinyl ester as the essential components. When those resins are used, because of excellent adhesion and moisture permeability thereof, a high effect of preventing rust of the metallic substrate can be exhibited.
It is desirable that the thermosetting fluorine-containing resin to be used for the thermosetting powder coating composition of the present invention has a fluorine content of not less than 10% by weight. When the fluorine content is less than 10% by weight, a coating film having a sufficient weather resistance cannot be obtained.
Also it is desirable that a MFR of the thermosetting fluorine-containing resin is within a range of from 0.1 to 100 g/10 min when measured at 100xc2x0 to 180xc2x0 C. When the MFR measured at 100xc2x0 C. of a low temperature side is less than 0.1 g/10 min, meltability of the resin at kneading with other additives such as a pigment is not sufficient, and color developing property, weather resistance and impact resistance of the obtained coating film are inferior. When the MFR measured at 180xc2x0 C. of a high temperature side is larger than 100 g/10 min, a viscosity of the resin at melt-kneading is lowered and the melt-kneading is not sufficient, and thus color developing property, weather resistance and impact resistance of the obtained coating film are also inferior.
Further it is desirable that in the present invention, the thermosetting fluorine-containing resin has a glass transition temperature of from 45xc2x0 to 120xc2x0 C. When the glass transition temperature is less than 45xc2x0xc2x0C., there are problems that the obtained coating composition is agglomerated at coating or an amount of composition sticking on a pulverizer increases at a pulverization step, thereby lowering productivity. When exceeding 120xc2x0xc2x0C., melt-kneading property is lowered, thereby making it impossible to sufficiently disperse additives such as a pigment at the melt-kneading step.
In the present invention, from the viewpoint of prevention of corrosion and enhancement of weather resistance, it is preferable to solely use the above-mentioned thermosetting fluorine-containing resin containing no chlorine as a resin component of the coating composition.
As a curing agent, those which have been used for thermosetting powder coating compositions can be used. Examples thereof are, for instance, a blocked isocyanate compound, acid anhydride, polyamine compound, glycidyl compound, isocyanurate compound, polybasic acid, and the like.
In addition, various additives which are usually used for powder coating compositions, for example, a pigment, filler, ultraviolet ray absorber, leveling agent, flowability control agent, antioxidant, heat deterioration preventive agent, gloss control agent, static charge control agent, foam preventing agent, and the like may be blended optionally. It is preferable that those third components do not contain chlorine.
Examples of the pigment are, for instance, one or two or more of inorganic pigments such as titanium dioxide, iron oxide red, yellow iron oxide and carbon black; organic pigments such as phthalocyanine blue, phthalocyanine green, quinacridone red pigment and isoindolenone yellow pigment; extender pigments such as talc, silica and calcium carbonate; metal powders such as aluminum powder and stainless steel powder; mica powder; and the like. Also it is preferable that a rust inhibiting pigment represented by pyrophosphoric acid or tripolyphosphate is blended.
The thermosetting fluorine-containing resin powder coating composition can be prepared by dry-blending each component, melt-kneading and pulverizing in the same manner as in known preparation method of thermosetting powder coating compositions. As case demands, the third components to be blended may be previously mixed to the resin component and curing agent. An average particle size of the thermosetting fluorine-containing resin powder coating composition is usually not more than 400 xcexcm.
Examples of metallic substrate which can be inhibited from rusting by the method of the present invention are substrates of iron, aluminum, stainless steel, copper, zinc, alloys thereof such as stainless steel and brass, and the like.
The coating can be carried out, for example, with a commercially available electrostatic coating machine or fluidized bed coating machine. After coating uniformly, a coating film is formed by baking with a hot air oven, infrared ray oven, induction heater, or the like.
The metallic substrate may be previously coated with a primer. However according to the present invention, a sufficient rust preventing effect can be obtained by coating the above-mentioned thermosetting fluorine-containing resin powder coating composition directly to the metallic substrate without forming a primer.
From the viewpoint of adhesion and a rust preventing effect, it is preferable that the metallic substrate is previously subjected to treatments such as washing with water, degreasing, rust removal, surface preparation and chemical conversion.
Degreasing is carried out by alkali cleaning by dipping in sodium silicate, sodium phosphate or caustic soda or spraying thereof. Rust removing treatment is carried out by a method of spraying a blasting material such as alumina, silicon carbide, iron powder or glass onto a substrate at high speed. Surface preparation is carried out by dipping into an aqueous solution of surface modifier or spraying thereof. Chemical conversion treatment is carried out preferably by dipping in zinc phosphate, iron phosphate, chromate, chromic acid or phosphate chromate or spraying thereof.